Blood lactate responses to submaximal field exercise tests in thoroughbred horses

The associations between velocity during a standardized, submaximal 800-m exercise test and blood lactate concentration after exercise were investigated in Thoroughbred racehorses on sand and grass racetracks. Predicted lactate concentrations for each horse's exercise test velocity were calculated from the line of best fit derived from results at each racetrack. The repeatability of the differences between the measured and predicted blood lactate response to exercise was also investigated. Exercise tests were conducted at speeds ranging from 12.8 to 16.6 m/s. The variability of velocity within the exercise test was expressed as the coefficient of variation (CV) of the times for the four 200-m sections.On the sand track, the coefficient of determination for the lactate-speed relationship was highest for an exponential regression equation [lactate (mmol/L) = 0.08e(0.33x)(m/s);r(2)= 0.58, P< 0.05;n = 21]. Similar coefficients of determination were calculated for linear (r(2)= 0.56) and second order polynomial equations (r(2)= 0. 57). On the grass racetrack, the relationship was best described by a second order polynomial equation [lactate (mmol/L) = -0.87x(2)+ 28. 17x - 211.41;r(2)= 0.57, P< 0.05;n = 25]. The mean differences between the measured and predicted blood lactate concentrations in repeated tests were 1.9 +/- 1.8 (SD) and 1.8 +/- 1.7 ( SD) on the sand and grass racetracks. There were no significant associations between the velocity of the exercise and the CV on either racetrack. Differences between measured and predicted blood lactate concentrations, based on track-specific lines of best fit, have potential application in field studies of fitness in Thoroughbred horses.     

Maximal lactate concentrations in horses after exercise of different duration and intensity

Lactate kinetics in whole blood of horses was investigated after exercise of differing velocities and duration. The following categories of exercise were used: A: <11 m/second and >180 seconds (n=35), B: >11 m/second and <180 seconds (n=17) and C: <11 m/second and <180 s (n=10). The mean peak lactate concentration determined in horses in category A was 4.49 ± 2.21 mmol/1, in B, 16.32 ± 4.81 mmoVl and in C, 4.58 ± 1.59 mmol/l. While the maximum lactate concentrations in categories A and C were always found immediately after the exercise, the peaks in category B were measured between the first and tenth minute after exercise. Mean lactate concentrations measured at 2-minute intervals after bouts of category-B exercise tended to stabilize 3 to 10 minutes after exercise; however, mean lactate concentrations measured during the intervals before and after the peak value differed significantly. The lactate concentration returned to pre-exercise levels within 20 minutes after exercise bouts of category C, but remained above pre-exercise levels up to 60 minutes after bouts of category-A and -B exercise. It was concluded that, for an evaluation of lactate data after intensive anaerobic exercise, sequential blood sampling at 2-minute intervals for a period of up to 12 minutes after exercise is necessary. Less frequent sampling may be a reason for the often described irreproducibility of lactate concentrations in horses. After aerobic or mild anaerobic exercise, one sample is sufficient, but it has to be taken as soon as possible after exercise.     

Pharmacokinetics of detomidine administered to horses at rest and after maximal exercise

Reason for performing study: Increased doses of detomidine are required to produce sedation in horses after maximal exercise compared to calm or resting horses. Objectives: To determine if the pharmacokinetics of detomidine in Thoroughbred horses are different when the drug is given during recuperation from a brief period of maximal exercise compared to administration at rest. Methods: Six Thoroughbred horses were preconditioned by exercising them on a treadmill. Each horse ran a simulated race at a treadmill speed that caused it to exercise at 120% of its maximal oxygen consumption. One minute after the end of exercise, horses were treated with detomidine. Each horse was treated with the same dose of detomidine on a second occasion a minimum of 14 days later while standing in a stocks. Samples of heparinised blood were obtained at various time points on both occasions. Plasma detomidine concentrations were determined by liquid chromatographymass spectrometry. The plasma concentration vs. time data were analysed by nonlinear regression analysis. Results: Median back‐extrapolated time zero plasma concentration was significantly lower and median plasma half‐life and median mean residence time were significantly longer when detomidine was administered after exercise compared to administration at rest. Median volume of distribution was significantly higher after exercise but median plasma clearance was not different between the 2 administrations. Conclusions and potential relevance: Detomidine i.v. is more widely distributed when administered to horses immediately after exercise compared to administration at rest resulting in lower peak plasma concentrations and a slower rate of elimination. The dose requirement to produce an equivalent effect may be higher in horses after exercise than in resting horses and less frequent subsequent doses may be required to produce a sustained effect.     

Glycogen depletion patterns in horses performing maximal exercise

Muscle biopsy samples were collected from the left middle gluteal muscle of horses participating in competitive barrier trials. Twelve horses were biopsied the day before and within 30 minutes of completion of an 800 m barrier trial. A further six horses were sampled the day before, and within 30 minutes of, completion of a 1200 m barrier trial. Serial muscle sections were examined histochemically for myosin adenosine triphosphatase activity after acid preincubation, to demonstrate type I, IIA and IIB fibres. The glycogen content in the individual fibres was assessed using the periodic acid Schiff (PAS) reaction. Total glycogen in muscle was measured fluorimetrically after hydrolysis to glucose. Significant decreases in total muscle glycogen levels of 167.1 +/- 22.2 and 158.1 +/- 23.2 mmoles glucose units kg-1 (dry weight) (mean +/- standard error) occurred from pre- to post trial samples for horses in both 800 and 1200 m trials respectively. There were no significant differences between the decreases in total muscle glycogen from horses in either 800 m or 1200 m trials. In both 800 m and 1200 m trials there was a significant decrease in the percentage of type IIA and IIB fibres classified as having high PAS staining intensity, and a significant increase in the percentage of fibres classified as having medium staining intensity when samples taken after the trial were compared to those taken before the trial.     

Growth hormone secretion in relation to plasma lactate and glucose concentrations during a maximal treadmill exercise test in horses

Veterinary Research Communications -     

Plasma lactate and purine derivatives accumulation after exercise of increasing intensity in standardbred horses

The aim of the experiment was to study the relationship between plasma lactate and allantoin accumulation in horses undergoing five exercises differing in intensity and length. Twenty-five adult trotter horses were used (18 males, two castrated, and five females), housed in three training centers. The horses were assigned to five groups: slow trot, over 2000 m (Group 1); slow trot over 1600 m (Group 2); fast trot over 1600 m (Group 3); fast trot over 2000 m (Group 4); fast trot over 2400 m (Group 5). Plasma was obtained from blood sampled at rest, at the end of the bout of exercise and after 15 and 45 minutes from the end of the bout of exercise and analyzed for glucose, lactate, uric acid, free fatty acids (FFA) and allantoin concentrations. Accumulations of plasma lactate and allantoin (mmol/sec) were calculated as difference between end of exercise and rest and between 45 minutes sample and rest, respectively.Ranking the intensity of exercise using the lactate concentrations at the end of exercise, the level of exertion was highest for Group 3 horses and lowest for Group 5 horses (20.9 and 2.8 mmol/l, respectively). At the end of exercise, glucose concentrations were much higher for horses undertaking the more intensive exercise (Groups 3 and 4 compared to Group 2). FFA concentrations were highest at the end of exercise for Groups 2 and 3 and after 15 minutes for Groups 4 and 5. Plasma uric acid and allantoin concentrations peaked 15 and 45 minutes from the end of exercise, respectively, independently of exercise intensity. The relationship between accumulation of plasma allantoin (y, dependent variables) and lactate (x, independent variable) was non-linear: y=0.15−2.61^*x+68.3^*x² (r²=0.900; se=0.19). This suggests that allantoin accumulation could be used together with plasma lactate to calibrate the workload to muscle conditions to prevent muscle injury.     

Endothelin response during and after exercise in horses

The objective of the present study was to measure plasma endothelin-1 (ET-1) at rest and during exercise in the horse. Six healthy, Standardbred and Thoroughbred mares (5.3+/-0.8 years; 445.2+/-13.1 kg) which were unfit, but otherwise accustomed to running on the treadmill, were used in the study. Plasma ET-1 concentrations were measured using a commercially available radioimmunoassay kit. Horses performed three trials: a standing control (CON) trial where blood was collected from the jugular vein every minute for 5 min; a graded exercise test (GXT) where blood samples were collected at the end of each 1 min step of an incremental exercise test; and a 15 min submaximal (60% VO(2max)) steady-state exercise test (SST) where blood samples were collected 1 min before, immediately after, and at 2 min, 10 min and 20 min post-exercise. Plasma ET-1 concentration did not change (P>0.05) during the CON trial where it averaged 0.18+/- 0.03 pg/mL (mean+/-SE). Surprisingly, plasma ET-1 concentration did not change during the GXT trial where it averaged 0.20+/-0.03 pg/mL. There were no differences between the mean concentrations obtained in either trial (P>0.05). Plasma ET-1 concentrations were, however, significantly elevated (P<0.05) immediately following exercise and at 2 min post-exercise in the SST. Post-exercise plasma ET-1 concentrations returned to baseline (P>0.05) by 10 min of recovery. Together, these data may suggest that ET-1 concentrations are altered in response to an exercise challenge.     

Anesthetic, cardiorespiratory, and metabolic effects of four intravenous anesthetic regimens induced in horses immediately after maximal exercise

OBJECTIVE: To determine the anesthetic, cardiorespiratory, and metabolic effects of 4 IV anesthetic regimens in Thoroughbred horses recuperating from a brief period of maximal exercise. ANIMALS: 6 adult Thoroughbreds. PROCEDURE: Horses were preconditioned by exercising them on a treadmill. Each horse ran 4 simulated races, with a minimum of 14 days between races. Races were run at a treadmill speed that caused horses to exercise at 120% of their maximal oxygen consumption. Horses ran until fatigued or for a maximum of 2 minutes. Two minutes after exercise, horses received a combination of xylazine hydrochloride (2.2 mg/kg of body weight) and acepromazine maleate (0.04 mg/kg) IV. Five minutes after exercise, horses received 1 of the following 4 IV anesthetic regimens: ketamine hydrochloride (2.2 mg/kg); ketamine (2.2 mg/kg) and diazepam (0.1 mg/kg); tiletamine hydrochloride-zolazepam hydrochloride (1 mg/kg); and guaifenesin (50 mg/kg) and thiopental sodium (5 mg/kg). Treatments were randomized. Cardiopulmonary indices were measured, and samples of blood were collected before and at specific times for 90 minutes after each race. RESULTS: Each regimen induced lateral recumbency. The quality of induction and anesthesia after ketamine administration was significantly worse than after other regimens, and the duration of anesthesia was significantly shorter. Time to lateral recumbency was significantly longer after ketamine or guaifenesin-thiopental administration than after ketaminediazepam or tilet-amine-zolazepam administration. Arterial blood pressures after guaifenesin-thiopental administration were significantly lower than after the other regimens. CONCLUSIONS AND CLINICAL RELEVANCE: Anesthesia can be safely induced in sedated horses immediately after maximal exercise. Ketamine-diazepam and tilet-amine-zolazepam induced good quality anesthesia with acceptable perturbations in cardiopulmonary and metabolic indices. Ketamine alone and guaifenesin-thiopental regimens are not recommended.     

Plasma potassium and lactate concentrations in thoroughbred horses during exercise of varying intensity.

To investigate the effect of moderate to high intensity exercise of up to 6 min duration on plasma potassium and lactate concentrations, 6 Thoroughbred horses were studied using a treadmill at a 5 degree incline. Each test consisted of an 8-min standardised warm-up followed by an exercise bout at 8, 9, 10 or 12 m/sec. The horses were galloped at each speed for up to a maximum of 6 min or until signs of fatigue were present. The horses were then walked at 0 degree incline. Carotid arterial blood samples were taken during and after the exercise. At 8, 9 and 10 m/sec there was a general pattern of an initial rise in potassium to a peak around 1.5 min of exercise with the concentration then slowly decreasing. At 12 m/sec there was a continuous rise to a peak at the end of exercise in all horses. Immediately after exercise there was a rapid return (within 3-4 min) to the potassium concentrations recorded at the end of the warm-up period. Plasma lactate peaked around the end of exercise at all speeds. At the highest intensity of exercise the mechanisms for the re-uptake of potassium did not appear to be able to match the rate of efflux. In contrast, at less intense work loads, the rate of re-uptake appeared to be similar to or slightly greater than the rate of efflux. It is possible that a disturbance in this balance between efflux and re-uptake could result in a disturbance in normal neuromuscular function during exercise.     

Clenbuterol plasma concentrations after repeated oral administration and its effects on cardio-respiratory and blood lactate responses to exercise in healthy Standardbred horses

To evaluate the effects of clenbuterol on cardio-respiratory parameters and blood lactate relation to exercise tolerance, experimental horses performed standardized exercise tests on a high-speed treadmill before and after administration of the drug. Clenbuterol was administered in feed to six healthy Standardbreds at a dose rate of 0.8 micrograms/kg b.wt twice daily for 5.5 days. Each horse was tested twice, without and with a respiratory mask, during two consecutive days. One week elapsed between the baseline tests without drug and the tests with clenbuterol treatment (each horse served as its own control). The results show an unchanged heart rate response to exercise 2 h after the last clenbuterol administration. The blood lactate response and the arterial oxygen tension during exercise did not differ before and after drug treatment. The oxygen uptake as well as pulmonary ventilation relative to the work load performed was essentially unaffected. The arterial pH during exercise was significantly increased (P less than 0.05) following clenbuterol treatment. Plasma levels of clenbuterol were maximal 2 h post-administration with values between 0.45 and 0.75 ng/ml. The plasma half-life of elimination was 10.4 h (+/- 2.25 SD). In conclusion, clenbuterol did not cause any major effects on the cardio-respiratory and blood lactate parameters studied in healthy horses performing submaximal exercise tolerance tests.     

Determination of the lactate breakpoint during incremental exercise in horses adapted to dietary corn oil

OBJECTIVE: To determine lactate breakpoint of horses and test for effects of training and dietary supplementation with corn oil on that breakpoint. ANIMALS: 7 healthy Arabian horses. PROCEDURES: Horses received a control diet (n = 4) or a diet supplemented with 10% corn oil (4). A training program, which comprised two 5-week conditioning periods with 1 week of rest, was initiated. Submaximal incremental exercise tests (IET) were conducted before the first and after both conditioning periods. Blood samples for determination of blood lactate and plasma glucose concentrations were collected 1 minute before IET and during the 15 seconds immediately preceding each speed change. Data collected were fit to one- and two-slope broken-line models and an exponential model. RESULTS: Good fits were obtained by application of the broken-line models (adjusted R2 > 0.92) to blood lactate concentration versus speed curves. Lactate breakpoints increased 41% after training. After training, slope 2 and peak blood lactate concentrations were greater in the corn oil group, compared with controls. Mean blood lactate concentration at the breakpoint was not affected by training or diet. Plasma glucose concentration versus speed curves also fit the broken-line models, and glucose breakpoints preceded lactate breakpoints by approximately 1 m/s in the second and third IET. CONCLUSIONS AND CLINICAL RELEVANCE: Lactate breakpoints can be determined for horses, using blood lactate concentration versus speed curves generated during submaximal IET and may be useful for assessing fitness and monitoring training programs in equine athletes.     

Effects of N,N-dimethylglycine on cardiorespiratory function and lactate production in thoroughbred horses performing incremental treadmill exercise

In a crossover study, either a placebo paste or N,N-dimethylglycine was administered orally at a dose rate of 1.2 mg/kg twice daily for five days to six thoroughbred horses, with bodyweights ranging from 424 to 492 kg. Using previously determined regression equations for oxygen uptake (VO2) against speed for each horse, a standardised exercise test was given with speeds equivalent to fixed percentages of the maximum oxygen uptake (VO2max). The test consisted of two minutes at speeds equivalent to approximately 40 per cent and 50 per cent VO2max, and one minute at speeds that produced approximately 60, 70, 80, 90 and 100 per cent VO2max. During the last five seconds of each exercise stage, the values of VO2, carbon dioxide production (VCO2), heart rate, arterial blood and plasma lactate concentrations, arterial blood gases and pH were measured. Before and immediately after the exercise test, muscle biopsies were collected from the middle gluteal muscle to determine the muscle lactate concentrations. The administration of N,N-dimethylglycine produced no significant differences in any of the measured values, and it is concluded that the compound has no beneficial effects on cardiorespiratory function or lactate production in the exercising horse.     

Isoproterenol-induced maximal heart rate in normothermic and hyperthermic horses

The heart rate (HR) induced by maximal beta-adrenergic activation, which was elicited by infusion of isoproterenol, was studied in 8 healthy horses before (control) and after hyperthermia was induced by IV administration of 2,4-dinitrophenol (DNP). Isoproterenol was administered IV at 1.0 micrograms.kg-1.min-1 for 3 minutes, and the HR was determined during the final 30 seconds of the infusion. As the rectal temperature increased (P less than 0.001) from 38.2 +/- 0.1 C (mean +/- SEM; normothermic control) to 40.1 +/- 0.1 C at 60 minutes after DNP administration, the isoproterenol-induced HR also increased from 198 +/- 4 beats/min (control) to 214 +/- 4 beats/min (P less than 0.001). It appeared that the values of HR achieved with maximal beta-adrenergic activation were augmented by the hypermetabolic, hyperthermic state induced by DNP.     

Effect of creatine supplementation on muscle metabolic response to a maximal treadmill exercise test in Standardbred horses

The aim of the present study was to investigate the effect of creatine (Cr) supplementation on muscle metabolic response in connection with a maximal treadmill exercise test, known to cause a marked anaerobic metabolic response and adenine nucleotide degradation. First, 6 Standardbred trotters performed a standardised maximal exercise test until fatigue (baseline test). The test used was an inclined incremental treadmill test in which the speed was increased by 1 m/s, starting at 7 m/s, every 60 s until the horse could no longer keep pace with the treadmill. After this baseline test, the horses were separated into 2 equal groups. One half received a dose of 25 g creatine monohydrate twice daily, and the other group were given the same dose of lactose (placebo). The supplementation period was 6.5 days, after which the maximal treadmill exercise test was performed again. A washout period of 14 days was allowed before treatments were switched between groups and a new supplementation period started. After this second supplementation period a new maximal exercise test was performed. After supplementation with creatine or placebo, horses were stopped after performing the same number of speed steps and duration of exercise as they had in the baseline test. Blood samples for analysis of plasma lactate, creatine (Cr), creatinine, hypoxanthine, xanthine and uric acid concentrations were collected at rest, during each speed step and during recovery. The total blood volume (TBV) was also determined. Muscle biopsies for analysis of muscle metabolites (adenosine triphosphate [ATP], adenosine diphosphate [ADP], adenosine monophosphate [AMP], inosine monophosphate [IMP], creatine phosphate [CP], lactate [La] and glycogen) were taken at rest, immediately post exercise and after 15 min recovery. The results showed no significant increase in plasma Cr or muscle total creatine concentration (TCr) after supplementation with Cr. At the end of exercise ATP and CP concentrations had decreased and IMP and lactate concentrations increased in muscle in all groups. Plasma lactate concentration increased during exercise and recovery and plasma uric acid concentration increased during recovery in all groups. No influence could be found in TBV after supplementation with creatine. These results show that creatine supplementation in the dosage used in this study had no influence on muscle metabolic response or TBV.